[Keyboard] Atreus: Flip the middle two keys when PCBDOWN is set. (#6616)

Flipping the columns isn't enough for the Atreus keyboard, since these
two keys are distinguished by row on the same column electrically.

keyboards/atreus/atreus.h

@@ -25,8 +25,23 @@
 #endif
 
 // This a shortcut to help you visually see your layout.
-// The first section contains all of the arguements
-// The second converts the arguments into a two-dimensional array
+// The first section contains all of the arguments.
+// The second converts the arguments into a two-dimensional array.
+// In the PCBDOWN case we need to swap the middle two keys: k35 and k36.
+#if defined(PCBDOWN)
+#define LAYOUT( \
+  k00, k01, k02, k03, k04,           k05, k06, k07, k08, k09, \
+  k10, k11, k12, k13, k14,           k15, k16, k17, k18, k19, \
+  k20, k21, k22, k23, k24,           k25, k26, k27, k28, k29, \
+  k30, k31, k32, k33, k34, k35, k36, k37, k38, k39, k3a, k3b \
+) \
+{ \
+	{ k00, k01, k02, k03, k04, KC_NO, k05, k06, k07, k08, k09 }, \
+	{ k10, k11, k12, k13, k14, KC_NO, k15, k16, k17, k18, k19 }, \
+	{ k20, k21, k22, k23, k24, k36,   k25, k26, k27, k28, k29 }, \
+	{ k30, k31, k32, k33, k34, k35,   k37, k38, k39, k3a, k3b } \
+}
+#else
 #define LAYOUT( \
   k00, k01, k02, k03, k04,           k05, k06, k07, k08, k09, \
   k10, k11, k12, k13, k14,           k15, k16, k17, k18, k19, \
@@ -39,3 +54,4 @@
 	{ k20, k21, k22, k23, k24, k35, k25, k26, k27, k28, k29 }, \
 	{ k30, k31, k32, k33, k34, k36, k37, k38, k39, k3a, k3b } \
 }
+#endif